The present invention relates to means for ionizing gas samples for analysis in a mass spectrometer.
A mass spectrometer analyses a gas sample by bombarding it with a flow of electrons, imparting movement to the ions obtained in this way and distinguishing them according to their trajectory or velocity.
To increase the measurement sensitivity and resolution of the mass spectrometer it is beneficial for the gas sample to be strongly ionized.
In time-of-flight mass spectrometers, ions produced by the ion source are launched into the entry of a flight tube, in which they retain a constant velocity. The nature of the ions is deduced from the time of flight corresponding to each type of ion in the gas sample to be analyzed, which is detected at the outlet from the flight tube. This entails launching a packet of previously accelerated ions into the entry of the flight tube and marking the departure time of the packet of ions and the arrival times of the various ions at the other end of the flight tube.
It is therefore advantageous to generate packets of ions with the shortest possible duration and containing a maximum number of ions. This is achieved with a pulse mode ion source.
The ion sources usually employed in mass spectrometers include an electron gun including a source of electrons and at least one electrode for conditioning the flow of electrons to generate an appropriate flow of electrons directed towards a gas ionization area in which ions are formed which are acted on by at least one electrode for conditioning the flow of ions. The flow of electrons is generally directed towards the gas ionization area in a direction perpendicular to the direction of the flight tube of the mass spectrometer. This results in a large overall size and makes integration difficult. A relatively small quantity of ions is produced, which limits the sensitivity of the apparatus.
The problem addressed by the present invention is that of designing a new ion source structure for mass spectrometers which is more compact and more sensitive and which can easily be integrated with other components of a mass spectrometer.
To achieve the above and other objects, an ion source in accordance with the invention, for use in mass spectrometers, includes an electron gun having an electron source and at least one electrode for conditioning the flow of electrons to generate an appropriate flow of electrons directed towards a gas ionization area in which ions are formed which are acted on by at least one electrode for conditioning the flow of ions; at least one microchannel wafer is disposed in the flow of electrons downstream of the electrodes for conditioning the flow of electrons so that a pulsed secondary electron beam containing many electrons is generated from a pulsed primary electron beam containing relatively few electrons.
The microchannel wafers multiply the flow of electrons to intensify subsequent ionization of the gas sample. This significantly increases the sensitivity and resolution of the apparatus.
An additional electrode for dispersing the secondary electron beam to retain its temporal properties and improve its spatial properties can advantageously be placed downstream of the area occupied by the microchannel wafer(s).
This encourages further intensification of the ionization of the gas sample and therefore increases the sensitivity of the apparatus incorporating the ion source.
The gas ionization area is preferably between an upstream repulsion electrode through which the secondary electron beam passes and which retains the electrons by repelling the ions and a downstream acceleration electrode which attracts the ions.
Because of this feature, the ion source can be at the entry of and aligned with the axis of the flight tube of a time-of-flight mass spectrometer. This achieves better integration of the ion source and makes the apparatus more compact.
For the secondary electron beam to retain its temporal properties and remain dense, so that all the ions of a packet of ions enter the flight tube at substantially the same time, the ionization area is preferably in the immediate vicinity of the microchannel wafer(s).
The electron source can be a filament heated to an appropriate temperature to generate a flow of electrons by thermal emission, in the conventional manner. The primary electron beam is then pulse modulated by a deflector electrode.
Alternatively, the electron source is advantageously a field-emission cathode with micropoints producing a pulse modulated primary electron beam.
The invention finds one particular application in the production of time-of-flight spectrometers incorporating an ion source of the above kind.